1. Field of the Invention
The present invention generally relates to image forming apparatuses and particularly relates to image forming apparatuses in which latent images are formed using a multi-beam system.
2. Description of the Related Art
There is a trend of greater importance being given to compactness in digital multifunction copiers, particularly in low-end products. Conventionally, oblique incidence type systems are known as optical systems for achieving compactness.
FIG. 16 is a lateral view showing one example of an oblique incidence type system. In FIG. 16, a light emitting device 1601 that outputs a beam for forming a latent image, a photosensitive drum 1603 for holding the latent image, and a polygonal mirror 1602 for scanning the beam are arranged in respectively different positions in a height direction. The beam output from the light emitting device 1601 is obliquely incident on mirror surfaces (scanning planes) arranged on lateral surfaces of the polygonal mirror 1601. It should be noted that numeral 1602′ indicates an upper surface of the polygonal mirror 1602.
In oblique incidence type systems, distances from a center position (rotational axis) of the polygonal mirror 1602 to a center and an edge of the mirror surfaces are different (distances a and b). These differences in distance cause the position of the beam irradiated onto the photosensitive drum 1603 to fluctuate in the height direction.
FIG. 17 is a diagram for describing a difference between an actual scanning line formed on the photosensitive drum 1603 using an oblique incidence type system, and an ideal scanning line. A curved line indicated by a solid line 1701 is an actual scanning line. A straight line indicated by a dashed line 1702 is an ideal scanning line. In oblique incidence type systems, the scanning line suffers curvature in this manner.
A correction lens may be arranged on the beam path to mitigate the curvature. The correction lens is formed such that its refractive index to the height direction changes in response to the scanning position. Curvature is mitigated to a certain extent by the correction lens and high image quality is achieved.
However, in order to effectively mitigate curvature, it is necessary to take such measures as ensuring the processing accuracy of the correction lens and to regulate the optical system to a desired state. Since this incurs longer manufacturing times and increased manufacturing costs, such measures are unsuitable for low-end products.
In this regard, the same fluctuation as that in oblique incidence type systems can occur also in perpendicular incidence type systems in which the beam is caused to be incident perpendicularly with respect to the rotational axis of the polygonal mirror 1602. In an ideal perpendicular type system, the locus of the beam is present within the rotating plane of the polygonal mirror 1602.
FIG. 18A and FIG. 18B show single examples of an ideal optical system in which there is no significant shift in the rotational axis and an actual optical system in which there is significant shift in the rotational axis. FIG. 18A shows the ideal optical system. Since the rotational axis does not shift, even if the distance from the rotational axis to the reflection position on the mirror surface changes due to the rotation of the polygonal mirror 1602, the height of the irradiated point of the beam on the photosensitive drum 1603 is maintained uniformly. On the other hand, FIG. 18B shows how the rotational axis is arranged shifted out of position due to installation error. For this reason, the distance from the rotational axis to the reflection position on the mirror surface changes due to the rotation of the polygonal mirror 1602, and accompanying this, the height of the irradiated point also shifts undesirably. This shift is undesirable since it leads to a reduction in image quality. In high-end models in particular, installation error in the rotational axis is a problem that cannot be ignored.
FIG. 19 is a diagram showing one example of a method for correcting curvature due to line replacement. Here, description is given using FIG. 19 of a conventionally proposed correction method (Japanese Patent Laid-Open No. 02-050176, 2003-182146, 2003-276235 and 2005-304011). In FIG. 19, the dashed line indicates an ideal scanning locus L0. The solid lines are actual scanning loci L1, L2, and L3 respectively that pass within ±0.5 lines with respect to the ideal scanning locus.
Looking at both ends of the scanning loci, of the three scanning loci L1, L2, and L3, it is L1 that is closest to the ideal scanning locus L0. In scanning regions positioned in a central area of the scanning loci, it is L3 that is closest to the ideal scanning locus L0. And in regions positioned between the end portions and the central area, it is L2 that is closest to the ideal scanning locus L0. Accordingly, to achieve an ideal straight line, if different lines are replaced in a sub-scanning direction in order of L1 =>L2=>L3=>L2=>L1 in response to the scanning region, then a substantially ideal straight line can be achieved. Dividing a single scanning period into multiple regions and forming a desired image by selecting an actual scanning line for each region in this manner is referred to here as line replacement. Furthermore, a juncture between one region and another region is referred to here as a replacement point.
Although a substantially ideal straight line can be achieved by employing the above-described method, a kind of stepping occurs near the replacement points. The stepping (jaggies) is a phenomenon that occurs originating in resolution roughness of the image data and insufficient gradations.
Smoothing processes have been put forth as a technique for hindering jaggies from becoming conspicuous. Smoothing processes are methods in which pixels near replacement points are extracted by pattern matching focusing on the fact that replacement points can be determined depending on only the formation of the image data, and these pixels are replaced by pixel data prepared in advance.
FIG. 20 is a block diagram of a smoothing process circuit according to related art. A line buffer 2001 is a buffer that temporarily stores pixel data targeted for pattern matching. A pattern matching portion 2002 is a block that compares several sets of pixel data outputted from the line buffer 2001 and predetermined patterns. A pixel conversion portion 2003 is a block that replaces pixel data in response to a result of pattern matching. It is desirable that patterns of 1:2, 1:3, 1:4, and 1: (5 or more) lines or edges or the like are used as the predetermined patterns. This is because these patterns are images that tend to be recognized visually as jaggies.
FIG. 21 is a diagram showing one example of patterns used in matching. Patterns 2101 and 2102 are 1:2 patterns. Patterns 2103 and 2104 are 1:3 patterns. The formation of replacement points is different in the patterns 2101 and 2102. Similarly, the formation of replacement points is different also in the patterns 2103 and 2104.
FIG. 22 is a diagram showing one example of pixel data that has been replaced by a smoothing process. Pixel data 2201 corresponds to the pattern 2101. Pixel data 2202 corresponds to the pattern 2102. Pixel data 2203 corresponds to the pattern 2103. And pixel data 2204 corresponds to the pattern 2104. As is evident from FIG. 22, the darkness of pixels is changed near the replacement points. This hinders stepping from becoming visually conspicuous.
Incidentally, the method for correcting curvature of scanning lines shown in FIG. 19 is one type of interpolation process in the sub-scanning direction. However, shifting microscopic phases (dot forming positions in the sub-scanning direction) using interpolation is a highly difficult technique in terms of electrophotographic characteristics. In low-end products in particular, there is a tendency for uniformity in fine line thickness to be impaired easily due to poor tone reproducibility of very small dots.
FIG. 23 is a diagram for describing the difficulty of maintaining uniformity in the thickness of fine lines. The rectangles in FIG. 23 indicate single dots of PWM (pulse width modulation). Numeral 2301 indicates an ideal single dot width of a fine line. Numeral 2302 indicates one example of a line for which replacement has been applied. Due to replacement, it has become a fine line in which jaggies are conspicuous. Numeral 2303 indicates one example of a line for which a smoothing process has been executed to reduce the jaggies. It should be noted that the accuracy for reproducing very small dots greatly affects the image quality for the line 2303. Numeral 2304 indicates one example of a line for which a smoothing process has been executed to reduce the jaggies. In particular, since the reproducibility of very small dots is insufficient for the line 2304, added dots for smoothing around the replacement points are lost. This is undesirable since fine lines become blurred or too thin. Numeral 2305 indicates one example of a line for which a smoothing process has been executed to reduce the jaggies. In particular, since the reproducibility of very small dots is insufficient for the line 2305, added dots become undesirably fatter than the desired width. This is undesirable since fine lines appear fatter than the intended width.